1. Field of the Invention
The invention relates to a magnetic yoke for an induction crucible furnace having a rod-shaped core stack being suitable for carrying magnetic flux generated by a furnace coil of the induction crucible furnace, the core stack being enclosed on three principal surfaces thereof not facing the furnace coil by a body having a C or U-shaped cross section.
Such a magnetic yoke is disclosed in U.S. Pat. No. 3,704,336.
2. Description of the Related Art
A similar magnetic yoke for an induction crucible furnace is disclosed in Publication No. D ME/D 118 289 D of the firm ABB. That induction crucible furnace is suitable for inductive melting of cast iron, steel, light metal, heavy metal and alloys, with an operation taking place, for example, at frequencies of 125 to 1000 Hz in the case of a construction as a medium-frequency induction crucible furnace. A current converter is used in order to establish an alternating voltage of a specified frequency.
The active part of the induction crucible furnace is the furnace coil which has an interior that is lined with a ceramic crucible. The alternating current flowing through the furnace coil generates an alternating magnetic field which is carried by metallic charge material inside th furnace crucible and by an iron core stack of the magnetic yokes outside the coil. The alternating magnetic field induces eddy currents in the metallic charge material, i.e. electrical energy which is converted into heat. Due to the transformer principle, the furnace consumes power from the supply mains, so that with energy constantly being supplied, the charge material is caused to melt. The electromagnetic forces acting on the melt result in an intensive bath agitation which ensures a rapid heat equalization and material equalization.
The magnetic yokes are disposed on the outside of the coil in the form of individual stacks that are distributed with gap over the circumference of the coil parallel to the furnace axis. The iron core stacks of the magnetic yokes serve the purpose of carrying the alternating magnetic flux, as already mentioned above. In this connection, a path of low magnetic resistance should be offered to the magnetic flux. The path at the same time only causes low eddy current losses. The use of the magnetic yokes reduces the reactive power necessary as a consequence of the reduction in the magnetic resistance in the yoke region of the flux. In addition, the flux is prevented from entering the generally ferromagnetic supporting outer structural components of the furnace (furnace body with casing) and the heating of the latter by eddy currents is consequently prevented.
A further purpose of the magnetic yokes is the radial bracing of the core against the electromagnetically produced forces and the forces produced by the thermal expansion of the refractory crucible.
In modern high-power induction crucible furnaces having a power of lMW/tonne of content of metallic charge material, the production, amplification and propagation of soundwaves, in particular, also has to be avoided.
In order to be able to fulfil such tasks, the magnetic yokes themselves must satisfy certain requirements:
The material carrying the alternating flux must have a high permeability and low eddy current losses. The construction is normally of suitable thin, mutually electrically insulated transformer laminations havin high electrical resistivity.
The magnetic yokes must have a sufficiently high mechanical section modulus in the radial direction in the installed state in order to make it possible to transmit the entire bracing forces (up to 100 tonnes/m.sup.2 of coil surface and over) with as few support points as possible.
The magnetic yokes should be torque-resistant and the second moments of area in the longitudinal direction (against bending) and against torsion should be high, in particular, in order to avoid resonances.
The individual laminations of the core stack of the magnetic yoke must be pressed together sufficiently strongly in order to avoid vibrations of the individual laminations (otherwise noise generation and, under certain circumstances, a destruction of the insulation of the laminations with the risk of core burning also occurs.
The section modulus of the magnetic yokes in the radial direction is essentially determined by their radial extent, and for this reason the dimensioning of the core stacks often has to be larger than would be necessary for reasons of the actual purpose, which is the carrying or guiding of the magnetic flux. Such a solution requires comparatively high costs to achieve the necessary or desirable section modulus.
The above-mentioned necessary compression of the core stacks in the direction perpendicular to the individual laminations can be carried out in various ways: what is generally known is the use of clamping bolts which are passed through holes in the core stacks. Such a structure permits a sufficiently good clamping, in particular, if a sufficient number of such clamping points is used or if, with few individual clamping points, the clamping forces are distributed uniformly over the entire area to be clamped by suitably stiff cover sheets on both sides of the core stack. Such a structure has the advantage of permitting the clamping to be readjusted to the optimum value at any time, but it has the disadvantage of making a fairly large number of holes additionally necessary in the laminations. That results in additional cut edges which may a have burr formation, if the construction is not quite exact or, for example, if the punching tool is worn. That gives rise to the risk of an electrical contact between the iron laminations and the core burning caused thereby.
A further clamping method in which the individual aminations are clamped between two cover plates which are clamped by clamping elements is also generally known. For example, the stacks may be welded with the cover plates and clamping elements may be prestressed. That structure has the advantage of providing very simple lamination shapes having only straight cut edges which can easily be of burr-free construction. However, there are the disadvantages that a subsequent adjustment of the clamping pressure is no longer possible and the specific contact pressure is not uniform.
Meanwhile it has been found that, in addition to the eddy current losses which are caused by the alternating magnetic field extending predominantly parallel to the laminations, eddy current losses, which are appreciable in some cases, occur at certain points in the core stack in a positionally limited manner. In the gap between the furnace coil and the melt and also in the region of the alternating magnetic field in the melt, the magnetic resistance is constant over the coil circumference, i.e. in the azimuthal direction, and accordingly, the flux densities along the coil circumference are also constant and the flux lines extend continuously parallel to the furnace axis. On the other hand, with the configuration of the magnetic yokes at the coil circumference described above, regions having low magnetic resistance alternate with regions having high magnetic resistance (core stacks and gaps) in the yoke space of the field on the outside of the furnace coil. Accordingly, for the flux, regions of high magnetic conductance are disposed in parallel with those of very low conductance. In the external region of the coil, the flux consequently largely takes its path through the regions of high conductance, that is to say it is carried virtually exclusively in the core stacks. However, at the coil ends or cor stack ends it propagates in the circumferential direction in order to go over in the interior of the coil into flux density which is uniform in the circumferential direction. Under the circumstances, some of the flux in the end region of the core stacks emerges from the stacks at right angles to the stacking plane of the laminations. As a result, appreciable additional eddy current losses, which may result in local overheating of the core stacks and of the covering laminations, are generated in the end region of the core stacks. At correspondingly large powers, separate, expensive, additional cooling systems are necessary at such points.
It is accordingly an object of the invention to provide a magnetic yoke for an induction crucible furnace, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and which, despite an overall simple construction, has a high moment of inertia or second moment of area and torsional moment.